It is known that the power consumed in a polyphase electric network is the sum of the powers effectively consumed in each of the phases and polyphase power meters include for this purpose devices which enable electric parameters relating to a number of phases to be measured in order to determine the corresponding power.
A power meter has recently been proposed, for example, in U.S. Pat. No. 4,359,684, which includes an electronic measuring device which receives at its inputs a signal representative of the current in one phase with which it is associated and a signal representative of the supply voltage from this phase, and emits at its output a train of pulses the frequency of which is proportional to the power flowing through this phase.
In a polyphase meter operating on this principle a number of electronic measuring devices are employed, each associated with a respective phase the output pulses from which are then summed in order to obtain the measure of the total power by means of a suitable summation circuit.
Certain of these electronic measuring devices are designed in order to derive their input signals from the associated phase without the agency of voltage or current transformers. A signal representative of the current is in particular derived from the voltage drop at the terminals of a shunt connected in this phase. A signal representative of the voltage is obtained by means of a voltage divider and a supply circuit produces at the input to the measuring device a continuous voltage which is substantially stable with respect to the phase voltage without employing any transformer.
Thus the electronic measuring device and its associated supply circuit "float" with the phase voltage, a high-impedance circuit providing the electric continuity between this supply circuit and a voltage distinct from that of the phase in question, such as that of a neutral conductor.
Hence in a meter connected to a polyphase network the measuring devices and their supply are brought to voltages which may be very different from one another and fluctuate within rather wide limits with respect to one another. The result is that the summator of pulses at the output from these circuits must normally be galvanically insulated with respect to at least certain of them. Further it is desirable that its supply may be derived directly from the network without involving any costly voltage transformer. This supply must function whatever the number of phases of the polyphase network which are effectively under voltage, in order to account for the power which is flowing even through only one phase if the other phases of the network are interrupted.
It is in addition desirable to achieve this supply with a minimum of components.